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Re: Cannot unwind through MIPS signal frames withICACHE_REFILLS_WORKAROU

To: "Ralf Baechle" <ralf@linux-mips.org>
Subject: Re: Cannot unwind through MIPS signal frames withICACHE_REFILLS_WORKAROUND_WAR
From: "Kevin D. Kissell" <kevink@mips.com>
Date: Tue, 13 Nov 2007 23:49:38 +0100
Cc: "Franck Bui-Huu" <vagabon.xyz@gmail.com>, "Andrew Haley" <aph-gcc@littlepinkcloud.com>, "David Daney" <ddaney@avtrex.com>, <linux-mips@linux-mips.org>, "Richard Sandiford" <rsandifo@nildram.co.uk>, <gcc@gcc.gnu.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <473957B6.3030202@avtrex.com> <18233.36645.232058.964652@zebedee.pink> <20071113121036.GA6582@linux-mips.org> <cda58cb80711130514x16356ea3x4069616c9ee3caac@mail.gmail.com> <019e01c82602$f5463bf0$10eca8c0@grendel> <20071113150820.GB6582@linux-mips.org>
Sender: linux-mips-bounce@linux-mips.org
> > True, though it should perhaps be noted that currently it's only on 4KSc/Sd
> > systems (which I know you work on) where it's even possible for the stack
> > *not* to have exec permissions, since the classical MIPS MMU gives
> > execute permission to any page that is readable.
> 
> Disabling PROT_EXEC on a mapping tells the kernel it doesn't need to take
> care of I-cache coherency.  So it's somewhat beneficial even in absence of
> a protection bit in the actual TLB hardware.

That depends on just what the consequences of I-cache incoherence might be.
Without help from the MMU, the kernel cannot *know* that a given location
isn't in the I-cache, because a program can always compute a pointer-to-function
to an arbitrary address and dereference it successfully so long as the location
is readable.  If it's only the user who does this that will suffer as a result 
of
I-cache incoherence, one can argue that it serves him right.  But if it can 
screw
up the execution of the kernel, or other user processes, I think we have to 
assume the worst.

> Some of these performance optimizations are impossible because the kernel
> can't have definate knowledge that certain addresses have never entered the
> I-cache.

Sad but true.

            Regards,

            Kevin K.

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